1. Field of the Invention
The present invention relates to a secondary battery, and more particularly to a secondary battery having at least one element for compensating for and preventing volumetric expansion during charging/discharging of the battery, in order to prevent the electrode assembly from deforming due to rising internal pressure and to compensate for and prevent the expansion of the secondary battery case.
2. Description of the Related Art
Recently, compact and light electric/electronic appliances including cellular telephones, laptop computers, and camcorders are actively developed and produced. Such portable electric/electronic appliances house a battery pack for operation in a place where additional power supply is unavailable. The battery pack has at least one battery therein to output a predetermined level of voltage for operating the portable electric/electronic appliances for a period of time.
The battery pack currently adopts a secondary battery, which can be charged and discharged, in consideration of economic aspects. Typical secondary batteries include nickel-cadmium (Ni—Cd) batteries, nickel-hydrogen (Ni-MH) batteries, and lithium secondary batteries such as lithium (Li) batteries and lithium ion batteries.
Particularly, lithium ion secondary batteries have an operating voltage of 3.6V, which is three times larger than that of nickel-cadmium batteries or nickel-hydrogen batteries which are used in can cases as the power supply for portable electronic appliances. The lithium ion secondary batteries also have a large energy density per unit weight. As such, they are widely used in the industry.
Lithium ion secondary batteries usually use lithium-based oxide as positive electrode active materials and carbon material as negative electrode active materials. Lithium ion secondary batteries are generally classified into liquid electrolyte batteries and polymer electrolyte batteries according to the type of electrolyte. Batteries using a liquid electrolyte are referred to as lithium ion batteries, and batteries using a polymer electrolyte are referred to as lithium polymer batteries. Lithium ion secondary batteries are manufactured in various shapes, including cylinder, rectangular, and pouch shapes.
Lithium ion secondary batteries generally include an electrode assembly formed by winding a positive electrode plate coated with positive electrode active materials, a negative electrode plate coated with negative electrode active materials, and a separator positioned between the positive and negative electrode plates to prevent a short circuit and allow the movement of lithium ions only; a lithium ion secondary battery case for containing the electrode assembly; and an electrolyte injected into the lithium ion secondary battery case to allow the movement of lithium ions.
A method of manufacturing a lithium ion secondary battery is described as follows.
A positive electrode plate is coated with positive electrode active materials and is connected to a positive electrode tab. A negative electrode plate is coated with negative electrode active materials and is connected to a negative electrode tab. The positive and negative electrode plates are laminated together with a separator, all of which are wound to complete an electrode assembly. The main component of the positive electrode active materials is a complex oxide including lithium (Li). Lithium carbonate and cobalt oxide are mixed in a radio of 1.2:1 and baked at 400° C. to 1000° C. to form LiCoO2, which is generally used as the positive electrode active materials.
Thereafter, the electrode assembly is contained in the lithium ion battery case in such a manner that it does not escape from the case. An electrolyte is injected into the lithium ion secondary battery case, which is then sealed to complete a lithium ion secondary battery.
However, lithium ion secondary batteries have a problem in that, when they are repeatedly charged and used, gas can be generated and the internal pressure of the battery can rise accordingly. The case of the lithium ion secondary battery then expands (i.e., a swelling phenomenon) and the electrode assembly deforms.
The swelling phenomenon and the deformation of the electrode assembly are caused as follows: when the voltage of the lithium ion secondary battery rises above a reference value due to overcharging, lithium carbonate remaining in the positive electrode active materials (LiCoO2) decomposes and generates carbonate gas. The electrode assembly then expands and the internal pressure of the case of lithium ion secondary battery rises.
If the electrode assembly deforms heavily, a short circuit can occur between the negative and positive electrode plates of the electrode assembly.
Meanwhile, if the internal pressure rises excessively when the lithium ion secondary battery is used, the battery can explode.
In order to solve these problems, a safety device referred to as a “safety vent” has been introduced in Korean Registered Patent No. 10-0329562.
The safety vent is generally a part of the cap plate or case of a rectangular-type lithium ion secondary battery, which is substantially thinner that the substrate (for example, 40% or less of the thickness of the substrate), and is adapted to fracture, if the internal pressure of the battery rises, and prevent danger such as explosion.
However, the safety vent has a problem in that it can be easily broken by an external impact (e.g., a drop) because it is substantially thinner than the substrate.
Furthermore, if the internal pressure of the lithium ion secondary battery rises and the safety vent fractures accordingly, the internal sealing of the battery is released. The battery then cannot be used any longer.
While lithium ion secondary batteries have been discussed above, the present invention is not limited to lithium ion batteries.